The Curious Case of North American Horses

Few mammals in the history of the planet have had a story as strange and complex as that of horses in North America.

Undomesticated horses grazing a typical dryland shrub-steppe ecosystem
Undomesticated horses grazing a typical dryland shrub-steppe ecosystem (photo by Cynthia del Río / Unsplash)

Few mammals in the history of the planet have had a story as strange and complex as that of horses in North America. During the Late Pleistocene, horses were a common species. The extinction of horses and other megafaunal animals had a profound impact on ecosystems, contributing to the expansion of the bison population and altering the course of evolution. When European explorers and conquerors reintroduced horses to the continent thousands of years later, it had a similarly dramatic effect, contributing to the near-extinction of bison and the displacement of numerous native species. The legacy of these horses can still be observed on the landscape today, reminding us of the intricate and often unexpected ways in which species interact with and shape the world around them.

The Horses of Long Ago

For millions of years, two species of ancient horses roamed the North American continent, adapting to their respective ecological niches in different ways. The smaller species, Scott’s horse (Equus scotti), weighed between 600-700 lbs and was better adapted to grasslands. The larger species, the Mexican horse (Equus conversidens), weighed close to 1000 lbs and appeared to be more adapted to woodland environments. Scott’s horse had a range that extended from Alaska to Mexico, while the Mexican horse was limited to northern Mexico, Arizona, New Mexico, and Texas.

During that time, the North American landscape was inhabited by a diverse array of surprising animals, including camels, Jefferson’s ground sloth, stag-moose, and mastodons. Both horse species became extinct in North America between 6,000 and 12,000 years ago. Some species became extinct relatively quickly after the arrival of humans in North America, while others experienced a gradual decline but managed to persist for as long as 8,000-10,000 years before becoming extinct.

The gradual disappearance of species from large geographic areas can sometimes take centuries or even millennia before the animal becomes extinct. Take the woolly mammoth, for example. This iconic animal vanished from large areas of the continent long before its final extinction over four thousand years ago, which occurred on the remote Wrangel Islands of Alaska. The gradual decline of this creature, despite its slow pace, had a profound impact on the environment, resulting in lasting alterations to the landscape and ecosystems that are still evident today.

The ecology of the two native horse species that roamed the North American continent prior to human arrival remains a subject of mystery and conjecture. However, it is certain that both species competed with various other ungulates, large plant-eating animals, for the limited resources available on the land. Besides facing intense competition for food, these native horses were constantly threatened by a diverse array of predators, such as dire wolves, saber-toothed cats, and the formidable American lion. The methods by which the now-extinct horses of the late Pleistocene adapted to their ecological niches in response to competition and predation remain a mystery. The entire population of these wild horses were probably no more numerous than the current population of undomesticated horses in the wildlands of North America today, despite inhabiting a much larger area.

The arrival of humans on the North American continent had a profound impact on ecosystems, disrupting the delicate predator-prey balance that had existed for millennia. The demise of megafaunal plant-eating animals like Scott's horse, the Mexican horse, long-horned (Bison latifrons), steppe (Bison priscus) and ancient (Bison antiquus) bison along with the carnivores that pursued them, created a new niche in which the smallest bison species (Bison bison) thrived. This exponential increase in bison population resulted in significant ecosystem changes. Referred to as the "dwarf" bison to distinguish it from larger Ice Age cousins, Bison bison became a dominant "weed species" in the prairie eco-niche. They were swifter afoot, reproduced quicker, and required less forage than their giant competitors. The eventual dominance of the bison was due to retrenchment, not magnificence.

Human Influences

During the period of approximately eight to twelve thousand years between the disappearance of megafauna and the arrival of Europeans and their horses, droughts and locust outbreaks led to wildly fluctuating bison population numbers, estimated to range from 10 to 60 million animals.

In 1519, Spanish conquistador Hernan Cortez introduced European horses (Equus caballus) to North America. With a small force of Spanish soldiers and native allies, Cortez and his army were able to defeat the Aztec empire and conquer their capital of Tenochtitlan in 1521. This marked the beginning of Spanish colonization in the region, which had a profound impact on the ecology, history, and culture of Mexico and the rest of the Americas.

By the mid-sixteenth century, the population of European horses in Mexico had multiplied into the tens of thousands. In 1598, Spanish conquistador Don Juan de Onate embarked on a mission to establish Spanish control over the Pueblo Indians in present-day New Mexico. However, the Pueblo Indians rebelled against his cruel treatment and seized the conquerors' livestock. By the late 1600s, the Pueblos had already established an intertribal horse trade. During the early 1700s, the Comanches and Navajos obtained their first horses either from the Pueblos or through commercial intermediaries.

Prior to the introduction of horses by Europeans to the New World, Native Americans in the Great Plains relied on foot hunting to supplement their agrarian societies, specifically for hunting bison. Over centuries, the limitations of foot hunting prevented significant negative effects on the bison population. However, the arrival of horses served as the forefront of the European ecological invasion of the plains, leading to significant transformations in Native American tribes. These tribes transitioned from agrarian societies to equestrian nomads, becoming heavily reliant on bison for their survival. Numerous economic, cultural, and ecological factors contributed to the near-extinction of the bison, but horses were the engine that made it possible. They not only facilitated effective hunting but also competed with bison for limited water and forage resources.

Bison (Photo by Yannick Menard / Unsplash)

It wasn't until the late seventeenth century that European horses arrived on the Great Plains, almost two centuries after their introduction to North America. Most of the indigenous hunters on the Great Plains were relatively new to the grasslands. The emergence of these societies paralleled the rapid development of pastoral nomadism in the Central Eurasian grasslands among groups such as former Chinese farmers and Siberian forest hunters, between 2500 and 3000 years ago.

From 1820 to 1900, horses were regularly abandoned by indigenous people, Mexicans, and white settlers when they became inconvenient or burdensome. In his 1848 book 'Notes of Eight Years' Travels and Residence in Europe with his North American Indian Collection,' George Catlin wrote the following: 'The tract of country over which we passed, between the False Washita and this place, is stocked, not only with buffaloes, but with numerous bands of wild horses... There is no other animal on the prairies so wild and sagacious as the horse. This useful animal has provided great assistance to the Indians living on these vast plains, allowing them to hunt game more easily, transport their belongings, and undoubtedly serve them better and more efficiently than a larger and heavier breed. Vast numbers of them are also killed for food by the Indians, at seasons when buffaloes and other game are scarce.'

In his memoir, Ulysses Grant documented seeing an immense horse herd between the Nueces River and Rio Grande in Texas that extended for “As far as the eye could reach to our right, the herd extended. To the left, it extended equally. There was no estimating the number of animals in it; I have no idea that they could all have been corralled in the state of Rhode Island, or Delaware, at one time.”

Free-roaming horse herds in the interior of Nevada and other areas of the Great Basin were primarily established in the late 1800s from horses that had escaped from settlers. Horses were hunted for their meat and hides in the early 1900s. Although a comprehensive census was not conducted until 1971, estimates suggest that the population of undomesticated horses peaked at around two million in 1920.

By 1930, undomesticated horses had mostly vanished from the plains and were primarily found west of the Continental Divide, with an estimated population ranging from 50,000 to 150,000. These horses inhabited lands that eventually came under the management of the Bureau of Land Management and the U.S. Forest Service. By the 1950s, the population had decreased to around 25,000 horses due to aerial hunting and water hole poisoning. This led to the enactment of the first federal free-roaming horse protection law in 1959, commonly referred to as the 'Wild Horse Annie Act,' which prohibited the use of motor vehicles to capture free-roaming horses and burros.

Further protections were established, and the undomesticated descendants of the European horses were recognized and protected by Congress through the passage of the 1971 Wild Free-Roaming Horse and Burro Act. Currently, there are nearly a hundred thousand free-roaming wild horses on western rangelands. According to the Bureau of Land Management, the estimated appropriate number to prevent ecological damage is 27,000, although many believe that figure to be a significant overestimate. The Act has resulted in significant costs to taxpayers, including millions of dollars spent on wild horse roundups, adoptions, and horse care. Additionally, the Act has contributed to the accelerated deterioration of ecosystems inhabited by these horses.

In 1971, legislators could not fully anticipate the future consequences of passing the Act. As it becomes increasingly impossible to adopt out the large number of undomesticated horses removed from western rangelands each year (four to six thousand), the costs to the environment and taxpayers continue to increase.

Training a wild horse requires sturdy fences, an abundance of time, sufficient pasture and training space, a horse trainer, and financial resources for food, veterinarians, and transportation. An increasingly urban America has made the cost of acquiring everything needed to properly care for a wild horse higher than buying an already trained horse. While it can be a wonderful challenge for the right person, for most, it is disastrous.

Despite the Bureau of Land Management's well-deserved criticism for prioritizing the interests of the ranching community over environmental concerns, the agency is caught between the proverbial rock and hard place when it comes to managing free-roaming horses. They are compelled to implement an Act that conflicts with other mandated laws, while also dealing with inadequate funding and a changing demographic that often makes large-scale horse adoption infeasible.

Ecological Ramifications

Prior to the immigration of Euro-Americans to western North America, immense herds of wild ungulates were the hallmark of the Great Plains. West of the Continental Divide, large herds of buffalo or other wild plant-eating animals did not exist. Livestock grazing and undomesticated horses have been superimposed on ecosystems that didn’t evolve under continual, ubiquitous grazing pressure. The current abysmal condition of ecosystems and loss of topsoil is a testament to the continual overgrazing by cattle, sheep, and horses over the last 150 years.

Because of competition with many other plant-eating animals, as well as predation by a diverse assemblage of carnivores, the ancient Scotts horse and Mexican horse used the landscape much differently than their undomesticated free-roaming counterparts today. It is unlikely they remained in one location or were present in large numbers. Even though mountain lions have been documented preying on undomesticated horses, their effect is inconsequential. They are a low density, territorial, stalking/ambush predator whose favored habitat is highly dissected terrain and prey of choice being deer. Many wild horses may never encounter a mountain lion in their life. There are no modern-day predators of undomesticated horses that have an effect on their population. They can loiter and overuse areas near water, the most biodiverse areas in dryland habitats. Similar to domestic livestock, their concentrated overuse of large areas compacts soils and increases erosion, ultimately decreasing ecosystem productivity.

As seen from the reintroduction of wolves in Yellowstone National Park in 1995, the reintroduction of even a single species of carnivore into an ecosystem can have significant beneficial effects. Prior to their reintroduction, excessive grazing by elk resulted in little or no aspen and willow regeneration, despite the presence of mountain lions. The absence of these woody species led to the near disappearance in beaver populations, with only one beaver colony remaining in the Park in 1995.

A beaver dam built with willow cuttings
A beaver dam built with willow cuttings (Photo by Niklas Hamann / Unsplash)

These large rodents are both a keystone species and ecosystem engineers, creating habitat for a wide variety of species by constructing dams using willow and aspen cuttings. This activity enhances both biodiversity and abundance. Twenty-five years after the reintroduction of wolves, there are now nine beaver colonies in Yellowstone.

In contrast to mountain lions, wolves are a highly visible pursuit predator. Their presence prevents elk overgrazing of willow and aspen in the winter by dispersing their presence. Despite an increased elk population in the Yellowstone area today compared to when wolves disappeared from the Park in the 1930s, the ecosystem has become more biodiverse, primarily due to the impact of wolf predation on elk, moose, and deer. Small mammals, songbirds, amphibians, fish, and insects have prospered as a result of the reintroduction of wolves. Interestingly, there are now even more mountain lions in Yellowstone than before the wolf reintroduction.

Due to their current existence in desert-like environments that didn't evolve under continual heavy grazing, undomesticated horses are having dramatic and deleterious effects on rangelands.

Public land ranchers and undomesticated horses in western North America are the primary beneficiaries of modern western romanticism. Thanks to movies and popular fiction, most people can easily imagine a wild horse running in a natural landscape with mane flowing in the breeze. This imagery evokes the rugged, independent cowboy who cares about the land while being mistreated by the Federal government. The conservative media perpetuates this fantasy for ranchers, while the liberal media does the same for the horses.

The wild horse has become emblematic of untamed passion and has been widely referenced by songwriters and musicians. Songs like 'Wild Horses' by The Rolling Stones, with its refrain 'Wild horses couldn't drag me away,' echoed the freedom that many people sought during the era of 'free love' in the late 1960s and early 1970s. Another example is the album cover of 'Against the Wind' by Bob Seger & the Silver Bullet Band in 1979.

While the undomesticated horses that roam the wildland areas of North America may seem majestic and free, their presence continues to cause significant ecological damage. Unfortunately, this damage is overshadowed by the even more substantial harm caused by domestic livestock, which leads to habitat degradation, erosion, and soil compaction. Undomesticated horses, despite their beauty, have a significant negative impact on the environment.

In terms of vegetation and water on North America's wild public lands, undomesticated horses and domestic livestock act as hoarders, leaving limited resources for wildlife. These non-native species and their descendants lack significant natural predators and freely graze and roam, at the expense of other native species. While their dominance may appear impressive, their unchecked presence has serious ecological consequences, including overgrazing, soil degradation, displacement of wildlife, loss of biodiversity, and, in extreme cases, starvation and death of undomesticated horses.